The present invention relates to a laser module for processing optical information to record or read information recorded in an optical recording medium such as an optical disk or an opto-magnetic disk by using laser light, and more particularly to a laser module dealing with a plurality of wavelengths such as combination of DVD and CD as well as an optical head and an optical information recording and reproducing apparatus using the same.
In recent years, DVD (Digital Versatile Disc) having a recording density seven times as much as that of CD, starts spreading rapidly in the form of two wave corresponding drive by which a conventional CD can also be used. In order to maintain compatibility between DVD and CD, the optical head used in the drive is provided with laser modules, collimate lenses and object lenses respectively exclusive for CD and DVD, which increases the number of parts, complicates optical adjustment and gives rise to an increase in the cost.
In the meantime, development of blue laser having a wavelength of 410 nm becomes active and putting to practice thereof in the near future is expected. Therefore, from now on, it is necessary to conceive an optical head in correspondence with three wavelengths which is compatible with CD and DVD and can deal with also blue laser. However, when parts are assembled in correspondence with each of three wavelengths, it is anticipated that reduction in size and thin size formation of the apparatus become difficult and complicated optical adjustment results in a considerable increase in cost.
As a constitution in correspondence with reduction in size of an optical head for multiple wavelengths and promotion of reliability thereof, for example, in Japanese Patent Laid-Open No. Hei 10-21577, there is disclosed an example in which two or three semiconductor laser chips are pasted on a silicon substrate having a micromirror and light receiving elements to thereby constitute a module.
However, in the case in which two semiconductor laser chips are arranged, when one of the chips is aligned to an optical axis, the other of the chips becomes out of the optical axis and therefore, it is necessary to provide means for correcting chromatic aberration. Further, in the case in which three laser chips are integrated to a silicon substrate having light receiving elements, it is necessary to correct chromatic aberration of two pieces of laser light out of the optical axis. However, a burden on a polarizing diffraction grating or a focus lens becomes considerable and it is difficult to use the constitution into practice as the optical head. Further, an area of the silicon substrate having the three laser chips and the light receiving elements in correspondence with respective light sources, becomes large-sized, resulting in hindrance for reduction in size and price of the laser module.
For example, there is shown a silicon substrate arranged with three laser chips having a chip width of 0.25 mm in FIG. 4. A width of a recess portion (hereinafter, referred to as sink portion) on the substrate arranged with the chips becomes as large as 1.1 mm. Further, when light receiving elements for focusing and for tracking in correspondence with respective laser light are arranged on opposite sides of the sink portion, the width of the silicon substrate becomes as large as about 3.9 mm and the number of silicon substrates taken from a silicon wafer decreases and the constitution becomes expensive. In the meantime, it is extremely difficult to bond three laser chips in one silicon substrate proximately to each other since a material of a device of semiconductor laser having a wavelength of 410 nm is GaN and a material of a device of DVD or CD laser is GaAs which is different from GaN.